1. Field of the Invention
This invention relates to the measurement of irregularities in the cross-machine nip pressure profile of two nipped rolls in the manufacture of web material, such as paper. More specifically, this invention relates to the substantially continuous measurement, in the cross-machine direction, of the irregularities of the nip pressure profile. Still more particularly, this invention relates to the continuous, substantially cross-machine direction, measurement of irregularities, such as hardness, in a wound paper roll by nipping the wound paper roll with a roll having a signal generating strip extending spirally along its surface.
2. Description of the Prior Art
In the winding of an endless web of material, such as paper on the reel of a papermaking machine, slight non-uniformities in sheet thickness, or caliper, become significant when such variations are magnified by the hundreds or thousands of layers of the paper wound on the roll. Such non-uniformities can cause areas of the wound web to burst, or become wrinkled and creased. Further, these variations in uniformity can cause problems in the subsequent converting operations when the continuous paper web is slit and cut into sheets of a size suitable for their eventual use. Even then, if the variations in caliper in the final cut sheets are significant, they can cause problems in the operation of printing machines.
In the manufacture of paper on a papermaking machine, the operator has several tools available to reduce and even eliminate the variations in cross-machine caliper. For example, the calender rolls can be heated or cooled at specific axial locations to increase or decrease the pressure along their nip line of contact. Other adjustments can be made by applying mists of water to the web, changing the slice lip opening at the headbox to modify the basis weight of the web at localized positions, adjusting the dewatering process in the press section, for example, by making crown or deflection adjustments on the rolls in the press section, or heating cross-machine positions of the web by steamboxes.
No matter how the adjustments are made to correct undesirable variations in the paper caliper, such adjustments must be made based on a measurement of the paper web caliper as the web is being formed, pressed, dried or finished on the papermaking machine.
One instrument for measuring web hardness in a wound paper roll is shown and described in Pfeiffer, U.S. Pat. No. 3,425,267. Another apparatus for measuring roll hardness in a roll of paper as it is being wound is shown and described in Wolfer, U.S. Pat. No. 3,540,270.
In the Pfeiffer patent, the instrument is a roll hardness meter which is hand held and is applied by the back-tender in a papermaking machine to the paper roll as it is being wound on the reel. The operator must then manually identify the specific location of the non-uniformity along the longitudinal length of the wound paper roll and take corrective action.
In the apparatus shown in the Wolfer patent, a small roller is brought into frictional tracking engagement with the roll of paper as it is being wound and is then guided in a traversing path along the surface of the paper roll parallel to its axis of rotation. The small traversing roller is electrically linked with a signaling device which signals small variations in the surface profile of the roll of paper being wound, which variations are indicative of its hardness. Such hardness is a function of variations in the paper caliper.
The Pfeiffer instrument works well, but it can only be applied to the paper roll by a human operator and only at such locations on the roll and at such times as determined by the operator. Obviously, such times and locations will be randomly and irregularly determined. Just as clearly, any sort of record of such measurements with the Pfeiffer instrument must of necessity be infrequent in the cross-machine direction due to the speed which a human can apply the instrument to a given location on the roll and move to a plurality of successive, uniformly spaced locations longitudinally along the roll. Also, in view of the relative slowness of human movement along the face of the roll of paper being wound, which can exceed 10 meters, and considering the speed of the oncoming paper web, which can exceed 1200 meters/minute for some paper grades, the readings at one end of the roll will not be repeated until literally thousands of meters of paper have been wound onto the paper roll.
In the Wolfer patent apparatus, the roll which traverses the face of the paper roll being wound proceeds uniformly, but its traversing speed is limited and, as a consequence, the cross-machine profile measurements are not made across the width of the paper web in a section of the web approximating a short length of the web in the machine direction. In other words, the measurements do not constitute a cross-machine profile of the web, but, rather, they represent a series of individual measurements which, over a period of time, are taken at successive locations longitudinally along the roll surface.
Since this device must contact the paper roll surface, it may contribute to breaks in the web and in any case, must be retracted whenever the sheet is to be threaded. Further, the cross-machine traversing mechanism can hang up broke.